In the semiconductor and related industries such as flat panel displays, data storage and hard disks, the need for maintaining ultra-clean surfaces throughout multiple process and rinse steps becomes more and more rigorous as design geometries shrink and yield requirements, for reasons of competitive economy, grow. In response to this need, a multitude of cleaning and process technologies have emerged, one of which is the enhancement of both chemical process and rinsing steps with megasonic energy. It is typical to have each chemical step followed by one or more rinsing steps. This form of processing is generally carried out in a process tank located in a “wet station”, and the handling of the wafers is carried out by robotic devices, although hand placement of loaded cassettes is still a frequent practice. The conservation of chemistry, the minimization of the use and waste of rinse media (typically high purity de-ionized water) and the reduction of process and rinse times have all been enhanced by the use of ultrasonic energy operating in the one megahertz region and higher. In common technical usage, “megasonic” has come to be used for any sonic cleaning device operating between 0.7-3 million hertz. While the substrate materials differ significantly depending upon the produce being manufactured, in semiconductor manufacture the substrate is most commonly called a “wafer,” and for simplicity the tern wafer shall be used in this disclosure to represent all substrates in any point of manufacture.
Among the many patents in the area of this art, one of particular pertinence to the present invention is U.S. Pat. No. 5,148,823 to Mario Bran for “Single Chamber Megasonic Energy Cleaner.” The first claim of this patent describes a dump valve in the lower portion of a wafer-cleaning tank that includes “ . . . a moveable valve member; and a device for generating sonic energy . . . said device being mounted to move with said member.” Independent claim 4 also recites the concept of a tank having a dump valve in its lower portion, the bottom wall forming a moveable valve member of the dump valve. Independent claim 6 recites a wafer cleaning tank having a tube extending from side to side in open communication with the exterior of the tank, a structure that is not relevant to the present invention.
The kernel of the invention in the Bran patent is a wafer cleaning tank having quick dump valve comprised of a moveable bottom wall of the tank, and the megasonic transducer may be secured to the moveable tank wall. The patent does not describe the fact that, in order to quickly dump the liquid contents of the tank, the bottom plate must be thrust downwards at high speed and then, perforce, must decelerate even quicker to its lower stop position at the end of the actuator retraction. This arrangement has at least two inherent structural weaknesses. First, the transducer cannot withstand such severe movement, the transducer typically being formed of a plurality of brittle crystalline members secured with adhesives to a rigid transmitting material such as, but not limited to, aluminum, quartz, synthetic sapphire or stainless steel. The assembly also includes seals of various types to the bottom plate of the tank, and the whole assembly undergoes significant mechanical shock at each opening and closing movement of a cycle.
Second, those skilled in the art of fluid flow and cleaning will understand that it is desirable for the liquid being dumped to flow down the surfaces of the wafers in as laminar a mode as possible and with as little turbulence in the wafer vicinity as possible. One of the worst places to have turbulence is in the bottom of the tank, adjacent to the lower portion of the wafers, as heavy particles, loosened by the megasonic energy, and escaping from the cascading overflow stream (if overflow is provided) will settle to the bottom of the tank. In the Bran arrangement the bottom plate drops rapidly, causing the liquid in the tank to fall upon that bottom plate, rebound against the upper descending stream and produce instantaneous turbulence that propels contaminating particles, which had been resting on the bottom plate, back up into a region of contact with the wafer load.
Thus it is apparent that the prior art is ripe for improvement.